Cleanup: Remove GetFontTable from FontCache. (#12677)

This interface is no longer used, so does not need to be implemented.

Removes manual memory management with malloc/free.

src/tests/mock_fontcache.h

@@ -30,7 +30,6 @@ public:
 	uint GetGlyphWidth(GlyphID) override { return this->height / 2; }
 	bool GetDrawGlyphShadow() override { return false; }
 	GlyphID MapCharToGlyph(char32_t key, [[maybe_unused]] bool allow_fallback = true) override { return key; }
-	const void *GetFontTable(uint32_t, size_t &length) override { length = 0; return nullptr; }
 	std::string GetFontName() override { return "mock"; }
 	bool IsBuiltInFont() override { return true; }